Re: [PATCH] Re: Bignum performance (was: Shrinking the C core)

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

On 14.08.23 10:09, Ihor Radchenko wrote:
> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
> 
>> Hm, then maybe we can look at the disassembly of then benchmark in SBCL?
>>    Not that in the end the compiler is so smart that it optimizes a lot
>> of stuff simply away because it can prove that the result of the
>> computations cannot possibly be observed?
> 
> Sorry, but I do not know how to do it. Not familiar with CL.
> 
Ok.  I used the code from https://dataswamp.org/~incal/cl/fib.cl/fib.cl.

And the first thing that stares at me in this function:

(defun fib (reps num)
   (declare (optimize speed (safety 0) (debug 0)))
   (let ((z 0))
     (declare (type (unsigned-byte 53) reps num z))
     (dotimes (r reps)
       (let*((p1 1)
             (p2 1))
         (dotimes (i (- num 2))
           (setf z (+ p1 p2)
                 p2 p1
                 p1 z))))
     z))

is the declaration (unsigned-byte 53).

The declaration means we are lying to the compiler because Z gets bigger 
than 53 bits eventually.  And all bets are off because of the OPTIMIZE 
declaration.  The result is that everything is done in fixnums on 64-bit 
machines.

; disassembly for FIB
; Size: 92 bytes. Origin: #x700530086C                        ; FIB
; 6C:       030080D2         MOVZ NL3, #0
; 70:       040080D2         MOVZ NL4, #0
; 74:       0E000014         B L3
; 78: L0:   410080D2         MOVZ NL1, #2
; 7C:       E20301AA         MOV NL2, NL1
; 80:       EB030CAA         MOV R1, R2
; 84:       651100D1         SUB NL5, R1, #4
; 88:       000080D2         MOVZ NL0, #0
; 8C:       05000014         B L2
; 90: L1:   2300028B         ADD NL3, NL1, NL2
; 94:       E20301AA         MOV NL2, NL1
; 98:       E10303AA         MOV NL1, NL3
; 9C:       00080091         ADD NL0, NL0, #2
; A0: L2:   1F0005EB         CMP NL0, NL5
; A4:       6BFFFF54         BLT L1
; A8:       84080091         ADD NL4, NL4, #2
; AC: L3:   9F000AEB         CMP NL4, R0
; B0:       4BFEFF54         BLT L0
; B4:       EA0303AA         MOV R0, NL3
; B8:       FB031AAA         MOV CSP, CFP
; BC:       5A7B40A9         LDP CFP, LR, [CFP]
; C0:       BF0300F1         CMP NULL, #0
; C4:       C0035FD6         RET

Tada!